1. Field of the Invention
The present invention relates to a method of placing a work piece on a measuring device and to a measuring device.
2. Description of Related Art
Various kinds of measuring devices are used to measure dimensions or a shape of a product. For example, a coordinate measuring device is used to measure various point positions on a surface of a product. A surface characteristic measuring device (shape measuring device, profile measuring device, or surface roughness measuring device) is used to measure a fine shape, profile, or roughness of the surface of the product. A roundness measuring device is used to measure roundness of a cylindrical product, for example.
In these measuring devices, a work piece (measured object) is placed on a table (stage) for measurement. Then, a measurement of various portions of the work piece is performed while displacing a measurement head corresponding to each type of measurement relative to the table. Various formats are used for the relative displacement of the table and measurement head. However, in many measuring devices, the table is displaced and a highly accurate guide mechanism or bearing mechanism is employed in the displacement mechanism to ensure accuracy of the measuring device.
For example, in a roundness measuring device, a rotating table is used in order to rotate a work piece and measure a circumferential surface thereof. In order to achieve highly accurate rotation, the rotating table employs an air bearing (see Japanese Utility Model Publication No. H03-008969). In the air bearing of the rotating table, an air layer of between several μm and tens of μm (microns) is formed in a gap between a rotor (on a rotation side) and a stator (on a fixed side), and the rotor is floating supported with respect to the stator by rigidity of the air layer, achieving non-contact rotation. The rotor and the stator each have a pressure-receiving surface facing the pressure-receiving surface of the other with the air layer therebetween. A shape of each pressure-receiving surface greatly influences rotation accuracy, which is a major capability of the roundness measuring device. Accordingly, in order to achieve a high degree of rotation accuracy, geometric accuracy on a sub-micron order is required in the finish working of the pressure-receiving surfaces of the rotor and stator in the air bearing.
In recent years, the size of measured objects in measuring devices has been getting extremely large. For example, roundness measurement has been required for extremely large work pieces such as wind power generator rotors and water turbines. In order to place such large work pieces on a table of a measuring device, the work piece has been suspended by a lifting device such as a crane or lifter. However, fine movement cannot be expected in suspending an extremely large work piece from a crane, as compared to a situation in which a worker carefully places a small work piece on a table.
In other words, in the work of suspending the extremely large work piece from the crane, after the work piece is displaced to a vicinity above a table surface, the work piece must be lowered slowly so as to not cause a collision in order to prevent damage to the work piece and table. However, although a skilled worker may be capable of the fine movement described above, an ordinary worker may find the operation difficult. Moreover, a typical crane or lifter may be unable to set an adequately low speed, or movement may be intermittent. Given this background, when suspended from a crane or the like, collisions are likely to occur between the work piece and the measurement table. Also, in a case where a work piece having a large mass is suspended by a crane or the like, a collision with the work piece is also large, and there is a possibility that the guide mechanism or bearing mechanism of the measuring device, which are highly accurate but not resilient, may be damaged or the like.
For example, in a case where a work piece is loaded onto the above-described rotating table of the roundness measuring device, the load of the work piece is absorbed by the air layer between the rotor and stator. In this example, the loaded work piece is extremely large and is very heavy, and when loaded with impact, the load capacity of the air layer of the air bearing is exceeded, and the opposing surfaces of the rotor and stator may come into contact with each other. When such contact occurs, the opposing surfaces of the air bearing may be damaged, degrading the rotation accuracy. In addition, when significant damage (a galling) is caused by contact between solids during rotation, the rotation operation may itself become impossible. Several strategies have been suggested in response to these issues.
A device and method for transporting a work piece to a coordinate measuring device is disclosed in Japanese Patent Publication No. H02-062006. In this document, a transportable stage is installed above a stage of the measuring device. In addition, the transportable stage is displaced from below a measurer onto a displacement bed on a side of the measuring device, and in this state, the work piece is suspended from above by a crane and placed onto the transportable stage. Then, the transportable stage, together with the work piece placed on the transportable stage, is drawn onto the stage (i.e., below the measurer) and measurement is performed. With this configuration, a collision during suspension from the crane is suffered by the displacement bed on the side of the measuring device, and an effect on the stage of the measuring device can be avoided.
Japanese Patent Laid-open Publication No. 2003-045492 discloses a measuring apparatus that includes an anti-vibration table and is configured to prevent a work table from vibrating and damaging a work piece during transfer of the work piece. In this document, the configuration is such that when the work piece is transferred, the work table of the anti-vibration table can be drawn downward and locked. By locking in this way, when the work piece is transferred, the work table can be prevented from vibrating and damaging the work piece due to an external force, and the inherent vibration prevention performance can be utilized by unlocking the work table.
In the configuration disclosed by Japanese Patent Publication No. H02-062006, described above, even when the work piece suspended from the crane collides with the transportable stage during transport, the effect on the stage of the measuring device can be mitigated. However, the transportable stage and a mechanism to draw the transportable stage sideways are required, unavoidably complicating the structure. Moreover, the configuration disclosed by Japanese Patent Publication No. H02-062006 is not able to mitigate collisions between the transported work piece and the transportable stage. In addition, when a collision occurs between the transportable stage and the work piece, the transportable stage and the work piece may be damaged.
In the configuration disclosed by Japanese Patent Laid-open Publication No. 2003-045492, described above, the work table can be prevented from vibrating due to an external force and damaging the work piece while transporting the work piece. However, the configuration disclosed in this document is not able to mitigate collisions between the transported work piece and the work table. In addition, when a collision occurs between the work table and the work piece, the work piece and work table, as well as the measuring device, may be damaged.